Former national security adviser Michael Flynn. (Reynold/Epa-Efe/Rex/Shutterstock)

ON FRIDAY, special counsel Robert S. Mueller III brought criminal charges against a fourth defendant in the investigation into Russian election interference: President Trump's former national security adviser, Michael Flynn. With Mr. Flynn's guilty plea, Mr. Mueller's probe into possible collusion between the Trump campaign and the Russian government continues to draw closer to the White House. The plea reveals that senior transition officials were guiding Mr. Flynn as he sought help from the Russian ambassador in undermining the Obama administration's policies.

Mr. Flynn served as national security adviser for 24 days before he resigned over reports that he had misled administration officials over the nature of his contacts with Sergey Kislyak. According to the special counsel, Mr. Flynn misled FBI investigators, as well — the criminal offense to which he pleaded guilty. Mr. Flynn requested that Mr. Kislyak keep Russia from retaliating harshly against new sanctions imposed during the final weeks of the Obama administration. Also during the presidential transition, he asked Mr. Kislyak to withhold Russian support for a U.N. Security Council resolution condemning Israeli settlements. Mr. Flynn denied all this to the FBI.

Mr. Flynn's guilty plea includes an admission that he coordinated these conversations with Mr. Kislyak with officials on the presidential transition team — among them, The Post reports, Mr. Trump's son- ­in-law, Jared Kushner. In December 2016, Mr. Kushner pushed Mr. Flynn to seek the help of Mr. Kislyak and other diplomats in quashing the U.N. Security Council resolution, which the Obama administration had decided not to oppose. Mr. Flynn also conferred with a "senior official" before acquiring Mr. Kislyak's promise that Russia would not retaliate against the United States on the matter of sanctions.

In negotiating with the Kremlin before Mr. Trump's inauguration, the Trump transition team undermined the foreign policy of the sitting president. What's more, the Obama administration designed its sanctions against Russia as retaliation for election interference. Mr. Flynn apparently promised that the next administration would review those sanctions — meaning Mr. Trump's advisers sought to weaken the U.S. attempt to hold Russia accountable for its meddling.

Investigators must now determine, if they have not already, who else was involved in this plot and how much the president-elect himself knew. Any direct involvement by Mr. Trump would cast in an even more concerning light the president's pressuring of then-FBI Director James B. Comey to drop the investigation into Mr. Flynn, along with his subsequent firing of Mr. Comey.

Mr. Mueller's decision to file charges only over Mr. Flynn's lies to federal investigators, along with the defendant's guilty plea, probably indicates that Mr. Flynn has chosen to cooperate with the special counsel. As the investigation develops, it is of the utmost importance that Mr. Mueller be able to continue unimpeded in his work. Mr. Trump must understand that further efforts to interfere in the investigation would be politically ruinous. And members of Congress on both sides of the aisle must stand ready to make clear to the president that any pardon of Mr. Flynn or any other defendant, or the termination of Mr. Mueller's appointment, would be cause for congressional action.